the / means just that, match a forward slash
the ? then says that you don't have to match the preceding character, but it will try to if it's there
Finally the $ ties the match to the very end of the string

So, if your string ends with a / it will be matched, and replaced with another slash, with no net effect.

However, if your script doesn't end with a slash, the whole substitution still matches because the slash was optional. In this case all we're matching is the 'end' of the string. The substitution then replaces this (zero-width) match with the new slash, which has the net effect in inserting the trailing slash.

for futher reading I’d recommend ‘perldoc perlre’ and the excellent “Mastering Regular Expressions” published by O’Reilly (which is superb)